Understanding Turndown Ratio
In flow measurement, accuracy is everything. Whether you’re monitoring the flow of DEF in agricultural equipment, cryogenic liquids in aerospace, or hydrogen gas in energy applications, choosing the right flow meter comes down to more than just size or cost. One of the most critical—and often overlooked—factors is turndown ratio.
So, what is flow meter turndown, and why should engineers and technicians care? Let’s break it down.
What Is a Turndown Ratio?
Turndown ratio refers to the range of flow rates over which a flow meter can accurately measure. It’s typically expressed as a ratio—for example, 10:1, 100:1, or even higher—indicating how much variability in flow the meter can handle while still staying within its stated accuracy range.
Example:
A flow meter with a turndown of 10:1 and a maximum flow capacity of 100 GPM can accurately measure down to 10 GPM. Below that, accuracy may fall outside acceptable limits.
Why Does Turndown Matter?
In real-world systems, flow conditions don’t stay constant. A process may run at full flow during peak demand but ramp down significantly at other times. If your meter only performs well at high flow rates, you’re left with inaccurate readings—or worse, no readings—during low-flow conditions.
Turndown ratio directly impacts:
- Measurement accuracy
- Process efficiency
- Compliance (especially in custody transfer applications)
- Equipment selection and system design
How Is Turndown Different From Rangeability?
These two terms are often used interchangeably, but there’s a subtle difference.
- Turndown considers only the range over which the meter maintains accuracy.
- Rangeability refers to the full range over which the meter can operate, even if accuracy isn’t guaranteed at the extremes.
For practical purposes in engineering and procurement, turndown is the more useful metric—especially when compliance, billing, or safety are involved.
Typical Turndown Ratios by Flow Meter Type
Different flow meter technologies have different turndown capabilities:
| Flow Meter Type | Typical Turndown Ratio |
| Turbine Flow Meters | 10:1 to 20:1 (up to 100:1 in ideal conditions) |
| Coriolis Flow Meters | 100:1 or higher |
| Ultrasonic Flow Meters | 50:1 to 100:1 |
| Differential Pressure | 3:1 to 4:1 (without compensation) |
| Magnetic Flow Meters | 20:1 to 100:1 |
| Positive Displacement | 10:1 to 100:1 |
These numbers can vary based on application, fluid properties, and whether the meter is installed with accessories like flow straighteners or filters.
When a High Turndown Ratio Is Essential
Here are a few cases where high turndown ratios can make or break your application:
1. Custody Transfer
In custody transfer, accuracy is mandated by regulatory bodies or contract terms. A meter that can’t maintain its accuracy across a wide flow range may lead to billing errors or even legal disputes.
2. Batching Applications
If you’re dispensing variable batch sizes, your meter must perform accurately at both high and low flow rates.
3. Variable Demand Systems
In industries like water treatment or chemical processing, flow rates can vary by season, process phase, or even shift. A limited turndown meter might force you to oversize equipment or install multiple meters.
Turndown and Turbine Flow Meters
At Turbines, Inc., we specialize in turbine flow meters, which offer a turndown ratio typically between 10:1 and 20:1, with some models capable of up to 100:1 under the right conditions. These meters are ideal for custody transfer of clean, low-viscosity fluids like DEF, hydrocarbons, or cryogenic liquids.
To maximize turndown with turbine meters:
- Ensure the fluid is clean and free of debris
- Install the meter in a section of straight pipe (or use flow conditioners)
- Avoid operating at extremely low Reynolds numbers
A turbine meter performing within its designed flow range will deliver outstanding repeatability and accuracy.
How to Choose the Right Turndown Ratio
When evaluating a meter, always consider the minimum and maximum flow rates in your application. Select a meter with a turndown ratio that accommodates your full operating range—ideally with some headroom.
Pro tip: Don’t size the meter solely for peak flow. Oversized meters may underperform at low flow, giving inaccurate readings or even failing to register flow at all.